This invention relates to a ball-and-roller bearing and, more particularly, to an improvement in the life of ball-and-roller bearings that are to be used in transmissions and engines in automobiles, agricultural machines, construction machines and iron- and steel-making machines.
Various alloy steels are conventionally used in ball-and-roller bearings and dominant among them are low-alloy steels including SUJ 2, or bearing steel type 2 according to the JIS.
The environment of the use of bearings has recently become increasing hostile for various reasons such as the increase in the areal pressure of load due to the downsizing of bearings and the entrance of foreign matter into lubricant oils.
While various methods have been proposed to extend the life of ball-and-roller bearings, the one that is described in Unexamined Published Japanese Patent Application No. 55423/1989 is characterized by increasing the amount of retained austenite in bearing steels so that even if indentations occur on the raceway surfaces or rolling contact surfaces due to the presence of foreign matter in lubricant oils, subsequent rolling contacts will increase the curvature of indentation edges to lessen the concentration of stresses at those edges, thereby improving the life of the ball-and-roller bearings made of those steels.
However, retained austenite which is capable of retarding the occurrence of cracking by lessening stresses that would otherwise develop at indentation edges or around nonmetallic inclusions has the disadvantage of poor fatigue resistance. Thus, in order to further extend the life of bearings, it is essential to improve the fatigue resistance of retained austenite. One method that can be adopted to retain the above-mentioned satisfactory characteristics of retained austenite and yet improve its fatigue resistance is to effect "precipitation hardening" by causing fine carbide crystals to be precipitated in the retained austenite. To this end, steels as base materials of bearings must be made not of low-alloy steels such as SCr 420 which is commonly used today but of high-alloy steels which, in addition to Cr (chromium), contain carbide-forming elements such as Mo (molybdenum); furthermore, it is necessary to carburize those high-alloy steels so that fine carbide crystals will be precipitated.
A method that adopts this approach is described in Unexamined Published Japanese Patent Application No. 55565/1981; a low-carbon high-chromium steel as a cold workable steel is carburized so that fine carbide crystals will be precipitated in retained austenite to improve the moldability of the steel while insuring that it will have wear resistance comparable to or even higher than that of high-carbon chromium steels such as SK 11.
The steel described in Unexamined Published Japanese Patent Application No. 55565/1981 has improved wear resistance but it has not been given any consideration about the requirements to be satisfied before the steel can be used as the base material of ball-and-roller bearings, nor has it been given any consideration about the appropriate range of the content of retained austenite.
A further problem with this prior art steel is that it is subjected to gas carburization in which carbon is mainly diffused inward of comparatively thin grain-boundary areas of the Cr oxide layer in the steel. Therefore, if the Cr content of the steel is increased to 3.0 wt % or more so that a Cr oxide layer will form on the surface of the steel, carbides will be concentrated locally at the grain boundaries of a phase which was initially austenite, thus making it impossible to produce a uniformly fine-grained carbide layer.
Unexamined Published Japanese Patent Application No. 205063/1989 discloses a wear-resistant stainless steel part that has been produced by performing plasma-assisted carburization on a low-carbon high-chromium steel; the part is characterized by the presence of a large amount of fine-grained carbides in the carburized layer, which contributes to higher wear resistance than in the case where gas carburization is effected. However, this prior art still lacks consideration about the requirements to be satisfied for applying the steel to ball-and-roller bearings, nor does it give any consideration about the appropriate range of the quantity of retained austenite or an improvement in the rolling fatigue life.